Since optical communication is a technique that could play a central role in future broadband communications, the optical components used in optical communication systems are required to be higher in performance, smaller in size, and lower in price for widespread use of the optical communication. Optical communication devices (optical function element) using photonic crystals are one of the leading candidates for the next-generation optical communication components that satisfy the above-described requirements. Some of the optical communication devices have already been put into practical use, and an example is a photonic crystal fiber for polarized light dispersion compensation. Furthermore, recent efforts have had a practical goal of developing optical function elements such as optical multiplexers/demultiplexers used for wavelength division multiplexing (WDM) communication, electro-optical modulators for performing conversion between an electric signal and an optical signal, and optical switches for controlling optical on/off of light.
A photonic crystal is a dielectric object having a period structure. Usually, the period structure is created by providing the dielectric body with a periodic arrangement of modified refractive index areas, i.e. the areas whose refractive index differs from that of the body. Within the crystal, the period structure creates a band structure with respect to the energy of light and thereby produces an energy region in which the light cannot be propagated. Such an energy region is called the “photonic band gap (PBG)”.
Providing an appropriate defect in the photonic crystal creates a specific energy level (“defect level”) within the PBG, and only a ray of light having a frequency (wavelength) corresponding to the defect level is allowed to be present in the vicinity of the defect. A defect created in a point pattern can function as an optical resonator that resonates with light having the frequency, and a linear defect enables the crystal to be used as a waveguide.
As an example of the above-described technique, Patent Document 1 discloses a two-dimensional photonic crystal having a body (or slab) made by materials whose refractive index is higher than that of air and provided with a periodic arrangement of modified refractive index areas, in which a linear defect of the periodic arrangement is created to form a waveguide and a point-like defect is created adjacent to the waveguide. This two-dimensional photonic crystal functions as the following two devices: a demultiplexer for extracting a ray of light whose frequency equals the resonance frequency of the resonator from rays of light having various frequencies and propagated through the waveguide and for sending the extracted light to the outside; and a multiplexer for introducing the same light from the outside into the waveguide.
Patent Document 2 discloses a two-dimensional photonic crystal in which the body is divided into a plurality of areas and a waveguide passing through the plurality of the areas is formed with a different period and size of modified refractive index areas provided in each of the areas so that light is allowed to pass through the waveguide in each of the areas with a different frequency band. In this construction, light whose frequency is included in a frequency band for passing through the waveguide of a certain area (i.e. first area) and not included in the frequency band for passing through the waveguide of an area adjacent to the area (i.e. second area) cannot be propagated from a waveguide of the first area to a waveguide of the second area, and reflected in a boundary between these areas. A resonator which resonates with light of the above-described frequency is provided in the vicinity of the waveguide of the first area, so that the light of this frequency propagated through the waveguides is reflected in the above-described boundary and extracted by the above-described resonator even if the light passes through without being introduced into the resonator. Therefore, a multiplexing efficiency is enhanced.
Patent Document 3 discloses an optical switch, in which a current is not injected/injected to a two-dimensional photonic crystal having a waveguide formed by a method similar to that of Patent Document 1 so that light of a predetermined frequency can be controlled to propagate/not to propagate (or on/off of light) can be controlled. An operation principle of the optical switch is as follows. When the current is not injected, light whose frequency is within the PBG is unable to be present in the body and locked within the waveguide to propagate through the waveguide (i.e. on-state). When the current is injected, the PBG is changed in accordance with a refractive index change of the body, so that the light having the frequency leaks from the waveguide to its surrounding body and fails to propagate the waveguide (i.e. off-state). It is also possible to set an off-state when the current is not injected to the two-dimensional photonic crystal and an on-state when the current is injected thereto, by adjusting materials (or refractive index) of the body and the period of the modified refractive index areas.
[Patent Document 1] Unexamined Japanese Patent Publication No. 2001-272555 ([0023]-[0027], [0032], FIGS. 1, and 5-6)
[Patent Document 2] Unexamined Japanese Patent Publication No. 2004-233941 ([0050]-[0053], FIG. 3)
[Patent Document 3] Unexamined Japanese Patent Publication No. 2002-303836 ([0022], [0028]-[0043], FIGS. 3-4)